Index: file.c
===================================================================
--- file.c	(revision 20868)
+++ file.c	(revision 20869)
@@ -1083,6 +1083,15 @@
 	return p;
 }
 
+void io_pcb_preproc_board(pcb_board_t *pcb)
+{
+	int n;
+
+	/* make postproc able to detect layers with broken link */
+	for(n = 0; n < PCB_MAX_LAYER; n++)
+		PCB->Data->Layer[n].meta.real.grp = -1;
+}
+
 void io_pcb_postproc_board(pcb_board_t *pcb)
 {
 	gdl_iterator_t it;
@@ -1097,6 +1106,21 @@
 		}
 	}
 
+	/* check if we have layers that are not present in the group string (broken input) */
+	for(n = 0; n < PCB->Data->LayerN; n++) {
+		if (PCB->Data->Layer[n].meta.real.grp == -1) {
+			pcb_layergrp_t *grp = pcb_get_grp_new_intern(pcb, -1);
+
+			pcb_message(PCB_MSG_WARNING, "Broken input file: layer group string doesn't contain layer %ld\n(Trying to fix it by introducing a new intern copper layer)\n", n);
+			if (grp != NULL) {
+				pcb_layergrp_id_t gid = grp - PCB->LayerGroups.grp;
+				pcb_layer_move_to_group(pcb, n, gid);
+			}
+			else
+				pcb_message(PCB_MSG_ERROR, "Failed to add a new layer group - the board in memory IS BROKEN.\n");
+		}
+	}
+
 	pcb_layergrp_create_missing_substrate(pcb);
 
 	/* have to rebind all subcircuits because the layer stack was not ready
Index: file.h
===================================================================
--- file.h	(revision 20868)
+++ file.h	(revision 20869)
@@ -86,6 +86,7 @@
 pcb_arc_t *io_pcb_element_arc_new(pcb_subc_t *subc, pcb_coord_t X, pcb_coord_t Y, pcb_coord_t Width, pcb_coord_t Height, pcb_angle_t angle, pcb_angle_t delta, pcb_coord_t Thickness);
 pcb_pstk_t *io_pcb_element_pin_new(pcb_subc_t *subc, pcb_coord_t X, pcb_coord_t Y, pcb_coord_t Thickness, pcb_coord_t Clearance, pcb_coord_t Mask, pcb_coord_t DrillingHole, const char *Name, const char *Number, pcb_flag_t Flags);
 pcb_pstk_t *io_pcb_element_pad_new(pcb_subc_t *subc, pcb_coord_t X1, pcb_coord_t Y1, pcb_coord_t X2, pcb_coord_t Y2, pcb_coord_t Thickness, pcb_coord_t Clearance, pcb_coord_t Mask, const char *Name, const char *Number, pcb_flag_t Flags);
+void io_pcb_preproc_board(pcb_board_t *pcb);
 void io_pcb_postproc_board(pcb_board_t *pcb);
 
 /* This is the version we support.  */
Index: parse_l.c
===================================================================
--- parse_l.c	(revision 20868)
+++ parse_l.c	(revision 20869)
@@ -2525,6 +2525,8 @@
 	if (settings_dest != CFR_invalid)
 		conf_reset(settings_dest, Filename);
 
+	io_pcb_preproc_board(PCB);
+
 	retval = Parse(NULL, fcmd, conf_core.rc.file_path, Filename);
 
 	if ((settings_dest != CFR_invalid) && (retval == 0)) {
Index: parse_l.l
===================================================================
--- parse_l.l	(revision 20868)
+++ parse_l.l	(revision 20869)
@@ -382,6 +382,8 @@
 	if (settings_dest != CFR_invalid)
 		conf_reset(settings_dest, Filename);
 
+	io_pcb_preproc_board(PCB);
+
 	retval = Parse(NULL, fcmd, conf_core.rc.file_path, Filename);
 
 	if ((settings_dest != CFR_invalid) && (retval == 0)) {